This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for selecting detector channels to facilitate optimizing the reliability of CT x-ray beam tracking.
In a multi-slice CT system, movement of an x-ray beam penumbra over detector elements having dissimilar response functions can cause signal changes resulting in image artifacts. Opening system collimation to keep detector elements in the x-ray beam umbra can prevent artifacts but increases patient dosage. Accordingly, at least one known CT system utilizes a closed-loop z-axis tracking system to position the x-ray beam relative to a detector array.
For example, in at least one known CT imaging system, the X-ray collimator assembly includes movable cams that are configured to track the focal spot position and thereby more accurately position the X-ray beam on the active elements of the multi-slice detector. Specifically, at least one known CT system detects the X-ray beam profile on one end of the detector and utilizes this information to adjust the cam positions. Whereas another known CT system detects the X-ray beam profile on both ends of the detector, and utilizes this information to adjust the cam positions.
Using both ends of the detector improves the ability to position the X-ray beam for situations when the beam profile does not move rigidly across the detector. However, both systems become less reliable when the detector, the collimator, and the X-ray focal spot are not well aligned. For example, if the X-ray tube is not positioned properly, the X-ray beam will be uniformly displaced toward one side of the detector, i.e., either the A-side or the B-side. Moreover, if the X-ray tube is properly aligned, the ends of the collimator and detector may remain skewed with respect to one another and the beam profile will appear skewed. Mis-alignment between the X-ray tube, the collimator and/or the detector may affect the measurement of the Z-ratio, R, used in the tracking control loop. The Z-ratio, as used herein, is defined a ratio of a detector outer row signal to a detector inner row signal, for a set of detector elements at one or both ends of the detector. The Z-ratio is generally useful when the detector outer rows sample the penumbra, i.e. the drop-off in intensity at the edges of the X-ray beam. Therefore, if the signals received from the detector outer rows are in the umbra, i.e., the uniform intensity region, of the beam, the Z-ratio will approach a constant value and cannot be used to reliably determine the beam position.